Structural and electronic properties of ultrathin Mg films on Ag(1 0 0) have been investigated by means of metastable deexcitation spectroscopy (MDS), work function change (Deltaphi), electron diffraction and helium reflectivity (R-He) measurements. At room temperature Mg grows on Ag(1 0 0) following the Stranski-Krastanov growth mode. Successive annealing of a 10 ML thick Mg film leads to its almost complete desorption at about 390 K and to the formation of a Mg-Ag alloy. R-He and electron diffraction measurements indicate that for annealing temperatures between 480 and 570 K, the alloy orders with a c(2 x 2) periodicity. Further annealing at higher temperatures restores the initial Ag(1 0 0) surface. (C) 2003 Elsevier Science B.V. All rights reserved.